BRUSSELS — Belgium’s prime minister refused to accept the resignations of his justice and interior ministers Thursday despite increasing evidence of failures by intelligence and law enforcement to prevent this week’s suicide bombings by Islamic terrorists.

With at least one attacker at large and an unknown number of accomplices, police detained six people in raids around the Belgian capital Thursday night. In a Paris suburb, a man suspected of plotting an imminent attack also was detained Thursday, but the interior minister reported no apparent link with Tuesday’s bombings at the Brussels airport and subway or the Nov. 13 attacks on Paris.

Authorities lowered Belgium’s terror-threat level by one notch, although they said the situation remained grave and another attack is “likely and possible.”

Belgium had been on its highest alert ever after the bombings, which killed 31 and wounded 270.

“We don’t have to be proud about what happened,” Justice Minister Koen Geens said of the government’s failures to halt the attacks. “We perhaps did things we should not have done.”

Less than a mile from the bombed subway station, European justice and home ministers held an emergency meeting where they condemned the “terrorist acts” as “an attack on our open, democratic society.” They also urged the European parliament to adopt an agreement allowing authorities to exchange airport passenger data.

A manhunt continued for one of the Brussels airport attackers who was recorded on a surveillance video and had fled the scene.

Belgian prosecutors said Thursday night’s raids targeted central Brussels, Jette and the Schaerbeek neighborhood, where police earlier had found a huge stash of explosives and bombmaking material in an apartment used by the Brussels attackers.

Prosecutors declined to comment on reports from Belgian state broadcaster RTBF and France’s Le Monde and BFM television that a fifth attacker may also be at large: a man seen on surveillance cameras in the Brussels metro carrying a large bag alongside one of the suicide bombers. It is not clear whether that man was killed in the attack or is a fugitive.

Authorities drew a line between the Brussels bombings and the Nov. 13 attacks in Paris. Both appeared to have been carried out by the same Belgium-based Islamic State terrorist cell.

Prosecutors have said at least four people were involved in the Brussels bloodshed, including brothers Ibrahim and Khalid El Bakraoui, identified as suicide bombers. European security officials identified another suicide bomber as Najim Laachraoui, a suspected bombmaker for the Paris attacks.

Khalid El Bakraoui blew himself up on the train, while Ibrahim El Bakraoui and Laachraoui died in the airport.

It is clear that some of the Brussels attackers had been on the run from authorities in France and Belgium but were still able to hide in safe houses, assemble bombs and carry out linked attacks.

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